Maytansinoids are highly cytotoxic drugs. Maytansine was first isolated by Kupchan et al. from the east African shrub Maytenus serrata and shown to be 100- to 1000-fold more cytotoxic than conventional cancer chemotherapeutic agents like methotrexate, daunorubicin, and vincristine (U.S. Pat. No. 3,896,111). Subsequently, it was discovered that some microbes also produce maytansinoids, such as maytansinol and C-3 esters of maytansinol (U.S. Pat. No. 4,151,042). Synthetic C-3 esters of maytansinol and analogues of maytansinol have also been reported (Kupchan et al., 21 J. Med. Chem. 31-37 (1978); Higashide et al. 270 Nature 721-722 (1977); Kawai et al., 32 Chem. Pharm. Bull. 3441-3451 (1984)). Examples of analogues of maytansinol from which C-3 esters have been prepared include maytansinol with modifications on the aromatic ring (e.g. dechloro) or at the C-9, C-14 (e.g. hydroxylated methyl group), C-15, C-18, C-20 and C-4,5.
The naturally occurring and synthetic C-3 esters can be classified into two groups:    (a) C-3 esters with simple carboxylic acids (U.S. Pat. Nos. 4,248,870; 4,265,814; 4,308,268; 4,308,269; 4,309,428; 4,317,821; 4,322,348; and 4,331,598), and    (b) C-3 esters with derivatives of N-methyl-L-alanine (U.S. Pat. Nos. 4,137,230 and 4,260,608; and Kawai et al., 32 Chem. Pharm. Bull. 3441-3451 (1984)).
Esters of group (b) were found to be much more cytotoxic than esters of group (a).
Maytansine is a mitotic inhibitor. Treatment of L1210 cells in vivo with maytansine has been reported to result in 67% of the cells accumulating in mitosis. Untreated control cells were reported to demonstrate a mitotic index ranging from between 3.2 to 5.8% (Sieber et al., 43 Bibl. Haematol. 495-500 (1976)). Experiments with sea urchin eggs and clam eggs have suggested that maytansine inhibits mitosis by interfering with the formation of microtubules through the inhibition of the polymerization of the microtubule protein, tubulin (Remillard et al., 189 Science 1002-1005 (1975)).
In vitro, P388, L1210, and LY5178 murine leukemic cell suspensions have been found to be inhibited by maytansine at doses of 10−3 to 10−1 μg/ml with the P388 line being the most sensitive. Maytansine has also been shown to be an active inhibitor of in vitro growth of human nasopharyngeal carcinoma cells, and the human acute lymphoblastic leukemia line C.E.M. was reported inhibited by concentrations as low as 10−7 μg/ml (Wolpert-DeFillippes et al., 24 Biochem. Pharmacol. 1735-1738 (1975)).
Maytansine has also been shown to be active in vivo. Tumor growth in the P388 lymphocytic leukemia system was shown to be inhibited over a 50- to 100-fold dosage range, which suggested a high therapeutic index; also significant inhibitory activity could be demonstrated with the L1210 mouse leukemia system, the human Lewis lung carcinoma system and the human B-16 melanocarcinoma system (Kupchan, 33 Ped. Proc 2288-2295 (1974)).
Because the maytansinoids are highly cytotoxic, they were expected to be of use in the treatment of many diseases such as cancer. This expectation has yet to be realized. Clinical trials with maytansine were not favorable due to a number of side effects (Issel et al., 5 Cancer Treat. Rev. 199-207 (1978)). Adverse effects to the central nervous system and gastrointestinal symptoms were responsible for some patients refusing further therapy (Issel at 204), and it appeared that maytansine was associated with peripheral neuropathy that might be cumulative (Issel at 207).
Accordingly, targeting techniques to selectively deliver drugs to the target cell were employed. Both cleavable and non-cleavable linkers have been investigated for several drugs, but in most cases, including the case of maytansinoids, in vitro cytotoxicity tests have revealed that antibody-drug conjugates rarely achieve the same cytotoxic potency as the free unconjugated drugs. Thus, it has been generally accepted that for targeted delivery of maytansinoids to be effective, the linkage between the maytansinoid and the cell-binding agent must be cleavable.
Furthermore, in the area of immunotoxins, conjugates containing linkers with disulfide bridges between monoclonal antibodies and catalytically active protein toxins were shown to be more cytotoxic than conjugates containing other linkers. See, Lambert et al., 260 J. Biol. Chem. 12035-12041 (1985); Lambert et al., in Immunotoxins 175-209 (A. Frankel, ed. 1988), and Ghetie et al., 48 Cancer Res. 2610-2617 (1988). This was attributed to the high intracellular concentration of glutathione contributing to the efficient cleavage of the disulfide bond between an antibody molecule and a toxin. More recently, a conjugate of maytansinoids linked to the anti-Her2 breast cancer antibody TA.1 via the non-cleavable linker SMCC was shown to be 200-fold less potent than a conjugate of maytansinoids linked to TA.1 via a linker having a cleavable disulfide bond (Chari et al., 52 Cancer Res. 127-133 (1992)).
Thus, cytotoxic conjugates linked via disulfide-containing cleavable linkers have been sought. Shen et al. described the conversion of methotrexate into a mercaptoethylamide derivative followed by conjugation with poly-D-lysine via a disulfide bond (260 J. Biol. Chem. 10905-10908 (1985)). Preparation of a conjugate of the trisulfide-containing toxic drug calicheamycin with an antibody was also described (Menendez et al., Fourth International Conference on Monoclonal Antibody Immunoconjugates for Cancer, San Diego, Abstract 81 (1989)).
U.S. Pat. Nos. 5,208,020 and 5,416,064, the entire disclosures of which are expressly incorporated herein by reference, disclose cytotoxic conjugates comprising cell-binding agents linked to specific maytansinoid derivatives via cleavable linkers, such as linkers containing disulfide groups, linkers containing acid-labile groups, linkers containing photo-labile groups, linkers containing peptidase-labile groups, and linkers containing esterase-labile groups
U.S. Pat. No. 6,333,410 B1, the entire disclosure of which is expressly incorporated herein by reference, discloses a process for preparing and purifying thiol-containing maytansinoids for linking to cell-binding agents, and U.S. Pat. No. 6,441,163 B1, the entire disclosure of which is expressly incorporated herein by reference, discloses a one-step method for preparing cytotoxic conjugates of maytansinoids and cell-binding agents, wherein the linker is a disulfide-containing cleavable linker.
Furthermore, U.S. Pat. No. 5,208,020 teaches antibody-maytansinoid conjugates with non-cleavable linkers, wherein the linker comprises a maleimido group. However, the reference contains no experimental data demonstrating that such conjugates are effective to treat disease.
It has now been found, unexpectedly, that cytotoxic conjugates of maytansinoids and cell-binding agents linked via non-cleavable linkers are extremely potent, and in many cases have unexpected advantages over conjugates of maytansinoids and cell-binding agents with cleavable linkers.